An optical waveguide device is a device for modulating phase or intensity of a light or switching an optical path of a light by changing the index of refraction of a waveguide included therein. In the optical waveguide device, a waveguide having a high index of refraction is formed in a ferroelectric or semiconductor substrate to confine the light propagated therethrough, and electrodes for applying a voltage are formed on or in the vicinity of the waveguide. In operation, a predetermined level of voltage is applied to the electrodes to change the index of refraction of the waveguide, so that the light which propagates therethrough is modulated in phase or intensity. In the same manner, the optical path is changed over among a plurality of waveguides.
One type of a conventional optical waveguide device includes a substrate consisting of lithium niobium oxide (LiNbO.sub.3) which is a ferroelectric material having high electro-optical effects, first and second waveguides consisting of Ti-diffused regions formed in the substrate, and first and second electrodes formed above the waveguides through an insulation, respectively. The first and second waveguides have a predetermined length of parallel portions close to each other with a predetermined gap.
In operation, when there is no voltage applied across the electrodes, there occurs a so called mode coupling between the close portions of the two waveguides, so that some proportion of light is transmitted from the first waveguide to the second waveguide at the close portions, and some proportion of light is transitted from the second waveguide to the first waveguide. If the length of the close portions of the waveguides is selected adequately, almost 100% of the light transmits from one waveguide to the other at the close portions thereof. Such a state is defined as a cross-over state hereinafter.
When, a predetermined value of the voltage is applied to the electrodes, there is generated an electric field, so that the index of refraction of the substrate changes due to the electro-optical effects of the substrate, and the coupling state of the close portions of the waveguides changes. If the value of the voltage thus applied is selected to be adequate, mode coupling does not occur, so that light supplied to the first and second waveguides pass therethrough straightly. Such a state is defined as a straight through or bar state hereinafter.
According to the conventional optical waveguide device, however, there is a disadvantage in that the operation state thereof is not stable and is dependent on the polarization of the supplied light. This is an obstacle to development of optical switches.
Next, a conventional directional coupler type optical switch which is independent of the polarization of light will be explained. In the directional coupler type optical switch, the density of a Ti diffused waveguide region is controlled to have the same value in a complete coupling length of the switch in both the TE mode and the TM mode by adjusting a thickness of a Ti layer provided on a substrate prior to the Ti-diffusion.
There has been developed an optical switch having a stable operation performance independent of the polarization for a light having a wavelength of 1.3 .mu.m. On the other hand, it is theoretically possible to realize the polarization-independence of the switch in which the complete coupling length thereof has the same value in both the TE mode and the TM mode of the light of 1.55 .mu.m by adjusting the thickness of the Ti layer.
However, conditions of optical confinement in the waveguide are different among lights having different wavelengths at the same Ti density of the waveguide, so that conditions of fabricating the optical switch on which the complete coupling length thereof has the same value in both TE mode and TM mode are different between lights having wavelengths of 1.3 .mu.m and 1.55 .mu.m, respectively. Therefore, it is difficult to form optical switches, one of which is for a light of 1.3 .mu.m and the other is for a light of 1.55 .mu.m in the same substrate by the same fabricating process.
Therefore, a conventional optical switch includes a first switching element for a light of 1.3 .mu.m and a second switching element for a light of 1.55 .mu.m formed in different substrates, respectively. Such an optical switch has the disadvantage of high fabrication costs, because the switching elements must be formed in the different substrates.